A hard-of-hearing person may wear two behind-the-ear hearing aids; one behind each ear. One of the hearing aids (the transmitting hearing aid) may pick up an acoustic signal and convert it to an electrical signal that may be wirelessly transmitted to the other hearing aid (the receiving hearing aid). In each hearing aid, the electrical signal may be amplified and converted back to an acoustic signal which may be played into the corresponding ear of the wearer.
It is known to communicate wirelessly between transmitting and receiving hearing aids by means of magnetic induction. A coil in the transmitting hearing aid may generate a magnetic field that passes through the wearer's head to the receiving hearing aid which has a receiving coil.
It is desirable for the transmitting hearing aid to be able to communicate not only with the receiving hearing aid but also with other, non-body mounted devices, remote from the wearer, such as, for example, televisions, radios or telephones. Some such devices may be bandwidth “hungry”. Whilst magnetic induction is fine for hearing aid-to-hearing aid wireless communication, its short range capability (typically less than 1 m) and its limited bandwidth (typically somewhere in the region of 10 to 13 MHz) make it unsuitable for communicating wirelessly with remote, bandwidth “hungry” devices. In those circumstances, it is preferred to communicate using electromagnetic radiation in the radio spectrum, which performs much better from the bandwidth and range perspective, such as, for example, the 2.5 GHz ISM (industrial, scientific and medical) radio band. However, RF (radio frequency) signals in this band (and other bands) are absorbed by the head, which poses a challenge for hearing aid-to-hearing aid communication.
It is known that one body-mounted wireless device may communicate efficiently with another such device mounted on the same body when each device has its antenna arranged so that the direction of the electric (E) field vector of the RF signal emitted by the antenna is more or less normal to the surface of the body at the position where the device is mounted. In the case of hearing aids, this means the direction of the E field vector needs to be normal to the plane of the wearer's ear or, to put it another way, parallel to an axis extending through the wearer's ears. For an elongate, linear antenna, such as a monopole or dipole antenna, the current flowing in the antenna generates an E field vector whose direction is parallel to the antenna's longitudinal axis. Hence, if a linear antenna was to be used in a hearing aid, the longitudinal axis of the antenna would need to be arranged normal to the wearer's head. However, at an operating frequency of around 2.5 GHz, which equates to a wavelength, λ, of 12 cm, a linear antenna would need to be a minimum of around 6 cm long (½λ which, for a behind-the-ear hearing aid, would not be practical.
What is required is an antenna that is suitable for use in a body mounted wireless communication device, such as a behind-the-ear hearing aid, operating at radio frequencies.